There is a drawing of Napoleon on his deathbed and detailed measurements of the dead Frenchman's head.
Mr Ibbetson was the Commissary officer on the British colony of St Helena and the archive was taken to New Zealand by his son in 1864.
It was kept in a suitcase for years by the family who did not realise the full significance of the treasures until recently.
They took the suitcase to a charity valuation day at an auction house and the full importance of the collection became clear.
The diary is revealing as it gives a fascinating insight into Napoleon who is described as: " ... very corpulent, about 5ft 6 inches high."
It adds that he has "short brown hair, sallow complexion, broad shoulders and has at times a very ferocious countenance."
One entry reveals that despite being defeated at Waterloo and surrendering he still wanted to invade Britain – and arrogantly thought he would be welcomed.
Mr Ibbetson wrote on August 7, 1815: "Napoleon talks of invading England with 200,000 Infantry & 6,000 Cavalry, but was uncertain of the disposition of the people of England, whether they would be for or against him, as he was induced to believe a great number would join him."
Perhaps his confidence stemmed from his previous escape from the British when he left the Mediterranean island of Elba where he was exiled and returned to power.
In the diary written on the Northumberland, Ibbetson records the story of how Napoleon convinced the people of Grenoble to join him following his return to his homeland.
He wrote: "September 8th. Walked with the Admiral I asked him of Napoleon ... touched upon the subject of his leaving Elba.
"The Admiral said that when he Landed in France, with 600 men the people of Grenoble hesitated whether they would join him, upon which Napoleon opened his great coat, & said "kill your Emperor" this produced the desired effect they immediately joined him."
The archive is being sold by auctioneers Art and Object in Auckland, New Zealand, and is expected to go for almost £100,000.
Hamish Coney, from the saleroom, said: "The direct link to a recorded officer who played such an important role in Napoleon's life and whose connection is so close makes this collection unique.
"When we first saw the collection we organised a family conference and found out that it had always been kept in a suitcase.
"They were aware of it, but it had just been passed down the line and they just hadn't thought about it that much until they opened it earlier this year.
"When we catalogued it they were surprised to hear of their relative's voyage of discovery.
"A complete collection like this has never come up for sale in New Zealand or Australia. It is very exciting.
"Denzil Ibbetson was a talented artist and the Commissary officer on St Helena who married there and had a family. His son took it to New Zealand in 1864.
"The archive includes drawings and watercolours of people and places on the island, hair cut from Napoleon the morning after he died and other items.
"Collectors are very excited about the sale and we've had interest from around the world.
"The diary is of particular interest as it tells how Napoleon still planned his invasion of Britain."